Phenol associates in benzene to a certain extent to form a dimer. A solution containing
$20 × 10^{−3}$ kg of phenol in 1.0 kg of benzene has its f. pt depressed by 0.69 K. Calculate the fraction of phenol dimerised. $K_f$ for $C_6H_6 = 5.120 mol^{−1}$kg.